1. Field of the Invention:
This invention relates to a direct-coupling control system for a torque converter in an automatic transmission for vehicles and, more particularly, to improvements in a direct-coupling control system for releasing a lockup of the torque converter at a preferable timing at various gear shifting times when direct-coupling and hence locking up a torque converter over gear shifting stages of more than adjacent four speeds of an automatic transmission having not less than four forward speeds.
2. Description of the Prior Art:
In an automatic transmission provided with a torque converter, it is effective to direct-couple and hence lock up the torque converter so as to decrease the fluid slipping losses of the torque converter. It is desired to lock up from a low-speed stage to increase the efficency as much as possible. To this end, there has been proposed by the inventors of the present invention, a system capable of releasing the lockup of a torque converter by a sole switching valve at various gear shifting times when locking up the torque converter over not less than three adjacent speeds. However, in order to attempt to lock up the torque converter over first to fourth speeds by applying this system to an automatic transmission provided with four forward speeds, another switching valve is necessary, and the system has to accept a complicated and large-sized structure. The multiplication in the number of speeds in an automatic transmission is inevitable to attempt to improve the fuel consumption while maintaining the present driving performance. The transmission is developed to have the multiplication in the speeds such as four or five speeds. This multiplication means increasing the number of control valves. Therefore, since the number of the control valves for controlling the lockup of the torque converter is increased, the automatic transmission is greatly restricted in the installation space and production.